This invention relates to spatially fine magnets and a method of forming spatially fine magnetic structures on a substrate. The magnets may be used to detect the movement and position of the substrate. Such information on position is normally sought in relation to technical or industrial applications, such as motors, generators, actuators, positioners, or other moving or rotating machinery. The invention is directed to a method for forming a spatially fine magnets and to methods for utilizing the magnetic structure in determining position.
It is often useful in industrial applications to know the position of the rotor of a motor or generator. In rotary devices, a rotary encoder supplies this information. In like manner, it is often useful to know the position of a linear member, such as an actuator or positioning device. In linear devices, the information may be supplied through a scale placed upon the moving part of a positioner or actuator. In both rotary and linear applications, the moving parts usually include a scale or other device, such as an encoder. The surrounding stationary parts contain devices to read the scale. The information so gathered is then processed with known signal processing techniques, for a desired result.
This result may be phase information useful in electric power generation or use. It may be positioning information useful in a variety of ways from pick-and-place devices to accurate machine tools or coordinate measuring machines. An example of a linear device may be marks made on a glass scale, secured to a moving member of the device, with the position of the scale being sensed optically. Dirt and erosion of surfaces over time can obscure the marks, making this structure susceptible to erroneous readings.
Rotary encoders, such as optical encoders, suffer other disadvantages. Accumulated dirt and disruptions of the light source or the light path will prevent satisfactory operation of the encoder. Elaborate schemes have been devised to overcome some of the difficulties of optical reading. An example would be a rotary device that includes separate conductive parts that are sensed as a rotor rotates and generates signals via capacitive coupling. However, circuitry is required for each arcuate segment of the conductive pattern, and the resulting structure will either be very complex in construction or not very accurate in its resulting resolution.
A series of magnets mounted on a rotating plate is another solution that has merit. In U.S. Pat. No. 5,117,183, a continuous ferromagnetic disk with localized separate magnetic fields is used to sense the position of rotating distributor parts in an internal combustion engine. The localized magnetic fields are sufficiently strong to be detected and to be useful in the application. A complicated looped wire fixture is used to induce the separate magnetic fields in a predetermined pattern on the ferromagnetic disk. The fixture in this device, or in others similar in nature, must magnetize in a very precise pattern, and not outside the pattern. As a result, a very precise and expensive fixture is required for each magnetic configuration.
It would be desirable to provide an encoder or other device with a magnetic structure that provides spatially separate magnetic fields of the required strength, but without requiring a complicated fixture to induce such fields. It would also be desirable to provide a method for manufacturing a spatially fine magnetic structure of this type by means of a simple and inexpensive process.
The invention overcomes the difficulties of these devices by using a better magnetic solution, rather than an optical or electrical solution. In the invention, fine magnets are provided on a moving linear or rotary substrate. As the linear or rotary substrate moves, a stationary sensor senses the movement of the individual magnets, processes this information, and accurately reports on the movement, and thus the position, of the moving part. The sensor will desirably be, but is not limited to, a Hall-effect sensor or a magneto-resistive sensor, which senses the magnetic field of each magnet.
In a method of the invention, separate magnets are formed on a substrate that may be a plate or a shaft of a rotor. Alternatively, the substrate may be a surface attached to a linear device, such as the ways of a machine tool or a coordinate measuring machine. A sensing device, such as a Hall-effect sensor, is placed on a stationary surface adjacent to the magnets. When the magnets move, the sensor detects the magnetic field of each magnet. The information so gathered is then processed and used to detect the position of the underlying substrate.
Thus, the combination of the magnets and the sensor, along with signals so generated, is useful for determining the position of a rotor, when the magnets and sensor are used in the manner of a rotary encoder. In such an application, a series of magnets is arrayed in a circle, generally around the periphery of a substrate, such as a shaft or a substrate attached to a shaft. The magnets are arrayed in a pattern as desired, with separation and resolution per the design. A larger number of small, fine magnets and tight spacing may be employed for greater resolution. For less demanding applications, fewer magnets may be used with greater spacing between them. The resolution of the angular position of a rotary encoder will depend on the precision of the information made available from the interaction of the magnets and the sensor.
In like manner, if the magnets and sensor are arrayed in a linear fashion, the linear position of the magnets and their substrate may be determined. This information is useful in applications such as position sensing or on/off sensing. A series of magnets is thus placed in line on a way or a surface attached to the item whose position is desired. If greater precision is sought, fine, closely spaced magnets may be used to increase the resolution of the information generated by the interaction between the magnets and the sensor. Some applications may be less demanding, for instance, a device to sense whether the magnet position is consistent with an xe2x80x9conxe2x80x9d position or an xe2x80x9coffxe2x80x9d position. In these cases, fewer magnets and greater spacing may be employed.
Position sensing magnets may be made by using particles of magnetic material in a matrix of an organic resin, such as an epoxy resin. In order for the magnets to have greater magnetic strength and a lasting effect, it is necessary that they be made of magnetically hard or xe2x80x9cpermanentxe2x80x9d magnetic materials. Such materials desirably include, but are not limited to, samarium cobalt, neodymium iron boron and other magnetically hard materials. These materials desirably have a high BH product, and can be easily sensed by a sensor, such as a Hall-effect sensor. These materials should also have high intrinsic coercivity, that is, high resistance to demagnetization, in order that they can be used and re-used for long periods, that is, as a permanent magnet.
This technology differs from that used in magnetic ink character recognition (MICR), used to print checks and bank notes. MICR technologies generally use iron oxide particles in printer""s ink. The concentration of particles in the ink must be small in order for rapid, reliable, non-clogging printing. The technology also differs in that the iron oxide particles require a magnet on the MICR reading head to magnetize the particles as a check passes through the head. Only then can the reader detect the magnetic field from the ink, and translate its reading into characters.